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A style sheet is a list of spelling, punctuation and other points of editing, with decisions about each item. An organization might have a style sheet that applies to all its documents, or a publishing house might have a separate style sheet for each book or series of books.
Usually, a style sheet is backed by one of the major editorial style manuals, such as Associated Press (AP), Harvard Stylebook or Chicago Stylebook. So if you come across any point that isn't covered in the style sheet, you'll apply the manual's style. Whenever the local style sheet and the manual disagree, the local style sheet wins.
You may have also come across the term in reference to Web pages. Web developers borrowed "style sheet" to refer to a list of colors, typefaces and designs that can be embedded in a Web document or stored in one place for an entire site.